Tuneable UJT oscillator circuit

ABSTRACT

A relaxation oscillator with an offset control. A timing circuit including a charge storage device in the relaxation oscillator is energized from a normally constant voltage source and one or more variable voltage sources. The first source also establishes a reference signal in a discharge circuit in the relaxation oscillator. Changes in the voltage from the variable voltage source do not affect the reference signal, but do affect a timing signal from the charge storage device. As the discharge circuit discharges the charge storage device each time the timing signal exceeds the reference voltage, changing the voltage from the variable voltage source does alter oscillator frequency.

United States Patent 11 1 Sanderson 1 TUNEABLE UJT OSCILLATOR CIRCUIT[75] Inventor: Albert E. Sanderson. Carlisle. Mass.

[73] Assignee: lnventronics, lnc.. Carlisle. Mass.

[22] Filed: Sept. 24. I973 [21] Appl. No.: 399,921

Related U.S. Application Data [62] Division of Scrt No. 249.942. May 3.1972.

abandoned.

[52] U.S. Cl. 331/111; 84/444; 84/DlG. 18:

331/177 R 51] Int. Cl. 03k 3/28 [58] Field of Search .331/111. 177 R.177 V [56] References Cited UNITED STATES PATENTS 1/1971 Chandos 331/111OTHER PUBLICATIONS Popular Electronics, Nov. 1968. p. 60.

1451 Apr. 22, 1975 Prinmr Examiner-Siegfried H. Grimm Anornqv. Agent. orF [rm-Cesari and McKenna l l ABSTRACT A relaxation oscillator with anoffset control. A timing circuit including a charge storage device inthe relaxation oscillator is energized from a normally constant 7Claims, 2 Drawing Figures l j 166i 44 l 4s 17s COARSEI FINE NOTE 74SELECTOR n2 PITCH VARlATlON 2a 7 BUFFER OSCILLATOR 26 OUTPUT AMPLIFIERTUNEABLE UJT OSCILLATOR CIRCUIT This is a division, of application Ser.No. 249.942 filed May 3. I972 now abandoned.

BACKGROUND OF THE INVENTION This invention generally relates to tuningmusical instruments and more specifically to a tunable oscillator foruse in such instruments.

Conventionally. a person listens to a reference note and adjusts amusical instrument until its note seems consonant with the referencenote. Consciously. or not, the person tunes a note for a zero beat withthe reference note. usually at some harmonic of either one or both thenotes.

This type of tuning is possible because a diatonic scale is based uponmathematical relationships. In practice, however. pianos and otherstringed instruments do not follow mathematical rules. Harmonicsgenerated by a given note are more than integral multiples of thefundamental. This deviation. termed stretch. may be defined as thedifference between the measured and theoretical second harmonicfrequencies of a note. Stretch is significant. In a piano, for instance,the second harmonic note from a string averages 2.002 to 2.006 times thefundamental frequency. Thus. if the fundamental notes are tunedmathematically, stretch causes a piano to sound out of tune.

Therefore. pianos and similar instruments must be tuned differently. Thegeneral approach is a complex. iterative process in which a tuner triesto reduce errors to a minimum step-by-step. Basically. a piano tunerstarts tuning a piano in a temperament octave" by adjusting a first noteto a reference frequency. He adjusts the remaining notes in thetemperament octave by listening to harmonics of third, fourth and fifthintervals. For example. in striking an interval of a third with apreviously tuned lower note, the tuner adjusts the upper note whilelistening to the beat between the fifth harmonic of the lower note andthe fourth harmonic of the upper note. He assumes the properrelationship exists when he obtains a predetermined beat frequency.

Listening to these harmonics reduces errors at the fundamental frequencybecause the harmonics multiply any error in terms of actual frequencydifferences. That is, a 4 Hz error at the fourth harmonic representsonly a 1 Hz error at the fundamental. Also, the use of harmonicsinherently tends to compensate for piano stretch. However. the processis not perfect and the tuner usually checks the temperament octave byretuning it using different intervals to minimize the tuning errors.

Once the tuner completes the temperament octave, he tunes other notes bycomparing harmonics while playing octave intervals. He may, for example,listen to the beat between the fourth harmonic of a lower, tuned noteand the second harmonic of the upper note while adjusting string tensionfor the upper note. Lower notes are tuned similarly. although notnecessarily with octave intervals.

Each piano note has two or three strings. During the foregoingprocedure, the tuner damps out strings so only one string actuallysounds when a hammer strikes all the strings associated with that note.After the tuner completes the procedure. he must tune the other stringsfor each note by comparing either the fundamental frequencies of twostrings associated with a given note or the fundamental of one note anda harmonic of another note an octave away.

As may be apparent, however. the entire procedure requires that a notesustain long enough to enable the tuner to determine the beat frequency.Obviously, the longer the interval the note sustains. the moreaccurately the tuner can determine the beat frequency. In tuning. eachnote struck sounds until it dies out naturally or the key is released.By dying out I mean that the note can no longer be heard. In the bassregion. this poses no real problem. However. as the frequency increases.the time the tuner hears the note decreases. Hence. he cannot determinethe beat notes in the treble range with as much accuracy.

in the bass region. the beat frequency becomes very low. For example.the full tone frequency difference at the low end of a piano is lessthan 3 Hz. The desired beat frequency is a small percentage of thedifference in a semi-tone, so it is less than 1 Hz. This is a verydifficult beat frequency to detect.

Although there are several tuning aids. no one aid has wide acceptance.In one. a high frequency oscillator produces an output clock signal at aselected frequency. A series of frequency dividers and an octaveselector switch provide a means for generating a reference signal at aselected subharmonic frequency. The tuning aid combines this referencesignal and an audio signal representing the note being tuned either togenerate an audible beat note or to deflect a pointer on an indicatingmeter. Unfortunately. these aids lose accuracy as the tuned note comesinto frequency with the reference. when the beat rate decreases below 20Hz. the audible beat note becomes inaudible. Similarly. an indicatingmeter uses a frequency-to-current converter so the current level goes tozero at a zero beat. As the current approaches zero, the visualindication becomes less accurate. Both types of display. therefore. loseaccuracy at the very time it is most necessary.

In another unit. the tuner attaches a piezoelectric transducer to aparticular string or a sounding board to produce a correspondingelectrical signal that is applied to the vertical deflection plates of acathode ray tube. A selector switch. crystal controlled oscillator and aseries of frequency dividers generate a selected reference signal whichenergizes the horizontal deflection plates of the tube. In using thiscircuit. one apparently assumes. erroneously, that a piano generates aconstant, repetitive wave form. In fact, a piano string generates anextremely complex wave form with a fundamental and harmonic components.often of the same magnitude. but slightly out of tune with each other.Furthermore. many of the component frequencies are not necessarilyconstant in magnitude because a string vibrates in many modes, each withits own damping constant. These factors cause the waveform to changecontinuously. so the display is difficult to interpret.

Another problem relates to dynamic response. lnitially. the amplitude ofthe signal is sufficient to drive the display off the screen. As thetone dies out, the input to the vertical deflection plates falls belowthe minimum level necessary for generating a usable display. An obvioussolution is installing a variable gain amplifier to maintain the outputat a constant value. However, a circuit which provides satisfactoryresults over the wide range of conditions and waveforms which the pianogenerates is difficult to attain in practice. If

the variable gain circuit actually tracks the decay, it may follow thewaveform and provide a dc output signal. Therefore, this solution is notpracticable especially in'view of the non-linear parameters orconditions and the short interval for a readable display; This effectivedynamic range further complicates tuning because adjusting a stringwhile monitoring the display is very difficult.

Still another tuning aid receives the audio signal from a piano andgenerates a corresponding electrical signal to energize the blanking orZ axis circuitry ofa cathode ray tube. A circular generator energizes'Xand Y axis deflection plates with a reference frequency so the electronbeam describes a circle on the screen. lfa note is in tune with thereference. the audio signal blanks and unblanks the electron beam duringthe same part of each revolution to thereby display one arcuate segment.A second harmonic input signal produces two such arcuate segments; athird harmonic input-signal, three segments; and so forth. If a givennote is not exactly harmonically related to the reference, the segmentsrotate. The direction of rotation indicates whether the note is sharp orflat while the speed of rotation indicates the difference infrequencies. As notes in the upper piano produce a display with a numberof segments. the spacesbetween adjacent sectors diminish; and theabsolute frequency deviation which producesa persistent display tends todecrease. Furthermore, alternately blanking and unblanking the beamproduces an indefinite segment termination on the screen. When thefrequency deviation is small, the indefininte termination makes itdifficult to determine whether the edges of the beams are moving. Whennotes in the lower range of the piano are tuned. the tuner must try toadjust while the tuning aid responds to harmonics. since subharmonics ofthe reference frequency generate complete circles on screen.

Apparently. another reason professional piano tuners are reluctant touse prior aids is that each piano is tuned uniquely, so a generalizedtuning aid that responds to the fundamental frequency of the note beingtuned does not really help the-tuner. The unique quality of each pianostems from its construction, string length, wear on hammers, and myriadfactors. As a result, piano tuners continue to work conventionally anddo not place any significant reliance on mechanical aids.

Therefore. it is an objectof this invention to provide an-oscillatorcircuit foruse in a tuning aid which is readily adapted for tuning awide variety of instruments.

SUMMARY In accordance with my invention, a tuner selects a specific notein an octave and a specific octave on the tuning aid. This selectiondetermines a nominal frequency for a clocking signal from a master clockoscillator including a timing circuit with a charge storage device. Thetiming circuit establishes a nominal frequency in response to the noteselection. Comparison means in a discharge circuit discharge the timingcircuit whenever a timing signal exceeds a reference signal. A normallyconstant voltage source establishes the reference signal in thedischarge circut and other means connect the constant voltage source toa timing circuit input terminal. One. or more variable voltage sourcesalso connect tothe timing circuit input terminal. When the voltage fromthe variable voltage source BRIEF DESCRIPTION OF THE DRAWINGS FIG. I isa blockdiagram of a tuning aid constructed in accordance with myinvention; and

FIG. 2 is a detailed schematic of an embodiment of a master clockoscillator circuit shown in FIG. 1.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT I r I. General Discussion 7 IAs shown in FIG. 1, my tuning aid 10 comprises an input circuit 12, areference circuit 14 and a detection circuit 16. The input circuit 12comprises a microphone ]8 which picks up signals generated as a musicalinstrument is tuned. For example, on a piano, it detects the soundemanating from astruck note. A conventional preamplifier 20 and anactive filter 22 isolate the signal being tuned from other signals whichthe microphone 18 serises (i.e., an active bandpass filter). The filter22 preferably is a tunable filter which has a quality fa'ctor greaterthan ten. Such bandpass filters are known in the art. The filter 22 itproduces an audio out-' put signal on a conductor 24'which connects'tothe detection circuit 16.

The reference circuit 14 produces'a second inputsignal to the detectioncircuit 16. A variable frequency master clock oscillator 26 constructedin accordance with this invention covers the twelve notes two octavesabove the highest octave to be tuned, for purposes which will becomeapparent later. A particular oscillator frequency is selected by'a noteselector 28which simultaneously tunes the active filter 22. An octaveselector 30 also controls the active filter 22 and further controls afrequency divider 32 which, in response to the signals from the masterclock oscillator 26, provides'a square waveoutput signal which is twicethe frequency determined by the note selector 28 and octave selector 30.That is, if the selectors 28 and 30 are set to select a musical A at 440Hz [hereinafter A(440.)], the filter 22 has a center frequency of 440 Hzwhile the master clock oscillator 26 generates a 28.16 kHz output and an880 Hz signal appears on the conductor 34 leading from the divider 32. j

The detection circuit I6 has a detector 36 which receives both the audiosignal on the conductor 24 and the reference signal on the conductor 34.It generates four output signals on output conductors 38-1, 387-2, 38-3and 38-4, Each output is a constant-amplitude, pulse-width-modulatedsignal with pulse width varying as a function of the phase differencebetween a note signal on the conductor 24 derived from the instrumentbeing tuned and a reference signal on the conductor 34, which is theoutput from the clock divider32. The pulse repetition rate is equal tothe selected reference frequency and the .rate at which the pulse widthchanges on each conductor depends on the frequency difference betweenthe note frequency and one-half the referency frequency, the pulses oneach conductor havingunvarying width if the struck note is in tune withthe reference. Low-pass filters 40 couple the pulse signals from thedetector 36 to a display 42. At any given time. a filtered ed dc outputis proportional to the width of an input pulse. If there is a frequencydeviation. each low-pass filter output varies from 0 to 200 percent ofits normal value at a rate which is proportional to the frequencydifference. a

The display unit 42 preferably contains one pair 0 lamps (e.g..light-emitting diodes) energized by each low-pass filter output.Mechanically. each lamp in a pair may be diametrically opposed in acircle. with adjacent lamp pairs separated by 45. As becomes apparentlater. the signals which energize lamps in space quadrature are 180' outof phase electrically. If a first lamp pair is at full brilliance. asecond lamp pair. displaced 90 from the first. is off. The lamp pairsthat are displaced i45 percent from the first are also off. for reasonsI discuss later.

When an incoming note is in tune. one pair of lamps may be at or nearlyat full brilliance or two pairs may be partially lit. However. therelative brilliance of the lamps does not change. As a result, thedisplay appears stationary. If there is a frequency deviation, theindividual lamp pairs reach full brilliance in one of two sequences. Ifthe note is shape" (i.e.. at a higher frequency than the reference).then the lamps reach full brilliance in a clockwise sequence; so thedisplay appears to rotate clockwise. When a note is flat, the sequenceis reversed and the display appears to rotate counterclockwise. As therepetition rate at which a given set of lamps reaches full brilliancedepends upon the frequency difference. the rate at which the displayappears to rotate indicates the magnitude of the deviation.

2. Specific Discussion The heart of this invention is in the manner inwhich the detector 36 and low-pass filters 40 condition input signalsand display the results. Still referring to FIG. 1, the signal themaster clock oscillator 26 and the divider 32 place on conductor 34 hastwice the frequency of the selected note. Division by at least two inthe divider 32 means that the outputs from the master clock oscillator26 must be four times the highest frequencies to be measured. In onespecific embodiment using a C as a lower octave limit and a 8" as anupper limit. the master clock oscillator 26 generates nominal signals inthe range between 16.744 and 31,609 Hz. Depending on the setting of theoctave selector 30, the clock divider 32 divides the oscillator outputby a faactor of 2" where l n 8. When the octave selector 30 is set forthe highest octave, the divider 32 divides the oscillator frequency by2. while a division by 256 occurs when the octave selector 30 is set forthe lowest octave. As a specific example. setting the note selector 28to A" causes the oscillator 26 to generate a 28.160 Hz signal. Thefrequency of the signal on the conductor 34 and the frequency which thetuning aid will sense are then as follows:

-Continued Signal On Frequency of Signal Octave Numher Conductor 34Being Measured 3 440 220 2 220 l l0 1 l It) 55 For the tuning aid to beeffective. there should be some provision to vary the frequency of themaster clock oscillator 26 shown in FIG. 1. The oscillator 26 generatessignals in accordance with the known mathematical relationships of theequally tempered scale. Coarse and fine pitch variation controls 44 and46 (FIG. 1) enable a tuner to vary the frequency of all the notes up toone-half a semi-tone in either direction (i.e.. an offset of i3 percentof the nominal oscillator frequency), while preserving the correctrelationship among the notes.

As shown in FIG. 2, the master clock oscillator 26 comprises aunijunction transistor in a relaxation oscillator circuit. Atemperature-compensating resistor 152 connects base 2" to a conductor154 from a power supply which provides a substantially constant voltage.An output resistor 155 is between "base 1 and ground. Two elements in atiming circuit generally control the nominal oscillator frequencyavariable capacitor 156 and a variable resistor 158. Thus. the voltagebetween base 1 and base 2 (E,,,,) is substantially constant.

To set the oscillator initially. the capacitor 156 is adjusted so thatthe oscillator 26 generates its highest required frequency. This is donewith the resistor 158 at a minimum value. Usually the resistor 158comprises a switched resistance ladder network which enables thefrequency for each setting of the note selector 28 to be adjustedindependently. During calibration. the frequencies are adjusted for thecorrect mathematical relationship. A buffer amplifier 160 couples thesignal from the output resistor 155.

The capacitor 156 and resistor 158 constitute two distinct means varyingthe frequency of the oscillator 26. In accordance with this invention.the oscillator 26 includes a third means for independently varyingfrequency over the offset range. As known, the unijunction transistor150 discharges when the voltage across the capacitor 156 (i.e., theemitter voltage) reaches a threshold which is a substantially constantpercentage (1 of (E,,,,) which establishes a reference voltage ('qE Thetime it takes the capacitor voltage to reach that threshold is afunction of the resistor and capacitor values and the voltage applied tothe timing circuit.

In the oscillator 26 in FIG. 2, this voltage appears across a capacitor166 and is equal to the voltage on the conductor 154 minus the voltageacross a resistor 162. The voltage across the resistor 162 depends onthe current through it and the current has two components. A firstcomponent is constant for a given setting of the note selector 28 anddepends upon the voltage from the constant voltage power supply on theconductor 154 and the series impedance of the resistors 162 and 158.

The second component is variable in response to the setting of the pitchcontrols which constitute two independent variable voltage sources. Aconductor 164 carries .this second component. As the pitch controlsincrease this current component, the voltage drop across resistor 162increases so the voltage across capacitor 166 decreases. As a result,the oscillator frequency decreases.

The remaining circuitry shown in FIG. 2 provides this variable secondcurrent component. A first resistor network constitutes a first variablevoltage source and comprises a resistor 172 for coupling the conductor164 to the wiper of a potentiometer 174, the potentiometer 174 beingenergized from the conductor 154. Variations in the position of thecoarse pitch control 44 offset the wiper arm. which constitutes anadjustable tap, from a normal position. Positioning the fine pitchcontrol 46 similarly alters the wiper arm on a potentiometer 176 alsoenergized from the conductor 154. A resistor 178 couples this wiper armto the conductor 164 and. together with the potentiometer 176,constitutes another variable voltage source.

The qualitative effect of varying either wiper arm position is the same.The component values are chosen so that a given physical displacement ofthe coarse pitch control 44 produces a large offset than the samedisplacement of the fine pitch control 46. Therefore. the followingdiscussion relates only to the operation of the coarse pitch control 44.

Two relationships exist in this circuit. First, as apparcm. the voltageon the conductor 154 is greater than the voltage on the conductor 164.Secondly, resistor 172 is at least an order of magnitude larger thanresistor 162.

Ma zero voltage offset position, there is a zero voltage drop across theresistor 172 so only the first current component flows through theresistor 162. If the coarse pitch control 44 is moved. the secondcurrent component from the conductor 164 changes the voltage across theresistor 162 and the capacitor 166.

Both pitch controls vary the frequency as a percentage of the basefrequency, so these controls can be calibrated in "cents" difference toraise or lower the resulting frequency. assuming that the oscillator iscalibrated with the ptentiometers 174 and 176 at their mid-points.

The tuning aid shown in FIG. 1 is sensitive and accurate. Tests shownthat the display has visible motion when the phase shift is less thanwith the accuracy being dependent upon the time the tuning aid sensesthe tone and the stability of both the tone and note. This means thatthe tuning aid senses a frequency difference which produces less than a10 phase shift over the interval the note signal exists. When operatedfrom a battery power supply, the tuning aid is very stable. Testsagainst a tuning fork show no displacement after 10 seconds of tone.This increased sensitivity and stability have enabled me to analyze howpianos are tuned conventionally.

Therefore, it is apparent that there are many modifications andalterations which can be made to my tuning aid, the specificallydescribed circuits and my method for tuning a piano. It is the object ofthe appended claims to cover all such variations and modifications ascome within the true spirit and scope of the invention.

What 1 claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A variable frequency oscillator circuit comprising:

A. a relaxation oscillator including i. a timing circuit including acharge storage unit and an input terminal for transmitting a timingsignal, and 7 ii. a discharge circuit connected to said timing circuitto receive the timing signal, said discharge circuit transmitting anoutput signal at a frequency nominally determined by the timing signalfrom said timing circuit, and including means for comparing the timingsignal and a reference signal,

B. a normally constant voltage source connected for energizing saiddischarge circuit to establish the reference signal,

C. means connecting said normally constant voltage source to said timingcircuit input terminal for providing a voltage thereto, and

D. a variable voltage source connected to said timing circuit inputterminal for transmitting a variable voltage thereto whereby saidvariable voltage source can offset the oscillator frequency from thenominal frequency determined by said timing circuit and said normallyconstant voltage source.

2. A variable frequency oscillator circuit as recited in claim 1 whereinsaid variable voltage source comprises i. a potentiometer including aresistance connected for energization by said normally constant voltagesource and tap means for obtaining a portion of the normally constantvoltage from said resistance means, and

ii. a resistor coupling said tap means circuit input terminal.

3. A variable frequency oscillator circuit as recited in claim 2wherein:

A. said relaxation oscillator discharge circuit comparison meansincludes a unijunction transistor, B. said timing circuit includes aresistor connected to said input terminal and in series with said chargestorage unit, the voltage across said unit constituting the timingsignal. said timing circuit resistor being variable to alter the nominalfrequency between discrete values. and C. said variable voltage sourcealtering the voltage at the timing circuit input terminal to produce anoffset from each of said nominal frequencies as a percentage of thenominal frequency. 4. A variable frequency oscillator circuit as recitedin claim 3 wherein said connecting means between said normally constantvoltage source and said discharge circuit comprises a resistor.

-5. A variable frequency oscillator circuit as recited in claim 1additionally comprising another variable voltage source connected tosaid timing circuit input terminal for transmitting another variablevoltage thereto, each of said variable voltage sources comprising:

i. a potentiometer including a resistance connected for energization bysaid normally constant voltage source and tap means for obtaining aportion of the normally constant voltage from said resistance means, andii. a resistor for coupling said tap connecting means to said timingcircuit input terminal, each of said potentiometers and each of saidconnecting resistors having different values of resistance. 6. Anoscillator circuit comprising: A. a relaxation oscillator including i. aunijunction transistor, with base electrodes and an emitter electrode,

ii. a timing circuit including an input terminal and a capacitor, thevoltage across said capacitor being coupled to said emitter electrode.

B. a normally constant voltage source connected to said base electrodes,

to said timing 7. An oscillator as recited in claim 6 additionallycomprising a second variable voltage source including a secondpotentiometer connected to said normally constant voltage source toprovide a variable voltage at a second adjustable tap and a secondresistor for coupling said tap and said timing circuit input terminal.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3:879'684 Dated p il 1975 Inventm-(S) Albert E. Sanderson It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 3, line 63, delete "circut" and insert circuit- Column 4, line31, delete "it" line 67, delete "referency" and insert reference Column5, line 4, after "filtered" delete "ed" line 26, delete "shape" andinsert --"sharp"- line 50, delete "faactor" and insert -factorline 51,delete l n 8" and insert --l" -'n 8--- Column 7, line 39, delete"ptentiometers" and insert -potentiometers-- Column 8, line 57, delete"potentiometers and each of said" Signed and Sealed this Twenty-seventhDay of July 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ofParentsand Trademarks Patent No.

Dated April 1975 Inventor(s) Albert E. Sanderson It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3,

Column 4,

Column Column 7,

Column [SEAL] line line

line

line

line line line line

line

delete "circut" and insert --circuitdelete "it" delete "referency" andinsert -referenceafter "filtered" delete delete delete delete "ed""shape" and insert "sharp"-- faactor" and insert --factor- "l n 8" andinsert l 'n =8- delete "ptentiometers" and insert -potentiometers--Attest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN' Commissioner ofPatents and Trademarks

1. A variable frequency oscillator circuit comprising: A. a relaxationoscillator including i. a timing circuit including a charge storage unitand an input terminal for transmitting a timing signal, and ii. adischarge circuit connected to said timing circuit to receive the timingsignal, said discharge circuit transmitting an output signal at afrequency nominally determined by the timing signal from said timingcircuit, and including means for comparing the timing signal and areference signal, B. a normally constant voltage source connected forenergizing said discharge circuit to establish the reference signal, C.means connecting said normally constant voltage source to said timingcircuit input terminal for providing a voltage thereto, and D. avariable voltage source connected to said timing circuit input terminalfor transmitting a variable voltage thereto whereby said variablevoltage source can offset the oscillator frequency from the nominalfrequency determined by said timing circuit and said normally constantvoltage source.
 1. A variable frequency oscillator circuit comprising:A. a relaxation oscillator including i. a timing circuit including acharge storage unit and an input terminal for transmitting a timingsignal, and ii. a discharge circuit connected to said timing circuit toreceive the timing signal, said discharge circuit transmitting an outputsignal at a frequency nominally determined by the timing signal fromsaid timing circuit, and including means for comparing the timing signaland a reference signal, B. a normally constant voltage source connectedfor energizing said discharge circuit to establish the reference signal,C. means connecting said normally constant voltage source to said timingcircuit input terminal for providing a voltage thereto, and D. avariable voltage source connected to said timing circuit input terminalfor transmitting a variable voltage thereto whereby said variablevoltage source can offset the oscillator frequency from the nominalfrequency determined by said timing circuit and said normally constantvoltage source.
 2. A variable frequency oscillator circuit as recited inclaim 1 wherein said variable voltage source comprises i. apotentiometer including a resistance connected for energization by saidnormally constant voltage source and tap means for obtaining a portionof the normally constant voltage from said resistance means, and ii. aresistor coupling said tap means to said timing circuit input terminal.3. A variable frequency oscillator circuit as recited in claim 2wherein: A. said relaxation oscillator discharge circuit comparisonmeans includes a unijunction transistor, B. said timing circuit includesa resistor connected to said input terminal and in series with saidcharge storage unit, the voltage across said unit constituting thetiming signal, said timing circuit resistor being variable to alter thenominal frequency between discrete values, and C. said variable voltagesource altering the voltage at the timing circuit input terminal toproduce an offset from each of said nominal frequencies as a percentageof the nominal frequency.
 4. A variable frequency oscillator circuit asrecited in claim 3 wherein said connecting means between said normallyconstant voltage source and said discharge circuit comprises a resistor.5. A variable frequency oscillator circuit as recited in claim 1additionally comprising another variable voltage source connected tosaid timing circuit input terminal for transmitting another variablevoltage thereto, each of said variable voltage sources comprising: i. apotentiometer including a resistance connected for energization by saidnormally constant voltage source and tap means for obtaining a portionof the normally constant voltage from said resistance means, and ii. aresistor for coupling said tap connecting means to said timing circuitinput terminal, each of said potentiometers and each of said connectingresistors having different values of resistance.
 6. An oscillatorcircuit comprising: A. a relaxation oscillator including i. aunijunction transistor, with base electrodes and an emitter electrode,ii. a timing circuit including an input terminal and a capacitor, thevoltage across said capacitor being coupled to said emitter electrode,B. a normally constant voltage source connected to said base electrodes,C. a resistor connecting said normally constant voltage source to saidtiming circuit input terminal, D. a variable voltage source including apotentiometer connected to said normally constant voltage source toprovide a variable voltage at an adjustable tap therein and a resistorcoupling said tap and said timing circuit input terminal.